Fungal pathogens can be lethal, especially among immunocompromised populations, such as patients with AIDS, or recipients of tissue transplantation or chemotherapy. Virulence factors for fungal pathogens, such as pathogenic fungal drug resistance, often are quantitative traits. A goal of our laboratories is to map quantitative trait genes that underlie the resistance of pathogenic yeast strains to anti-fungal agents and to determine the evolution of this resistance. This work will improve diagnosis and treatment strategies and lead to a better understanding of the evolution of adaptive complex traits. The preliminary data for mapping sensitivity of the pathogenic strain (YJM789) to one antifungal drug (Fluconazole) using whole genome tiling array successfully identified a candidate gene (PDR5), and subsequent experiments were able to show that the mutation is solely responsible for YJM789-sensitivity to Fluconazole (a Mendelian trait). Surprisingly, the same mutation confers YJM789 drug resistance to another antifungal drug (Amphotericin B, AmB), which is a quantitative trait also determined by other genetic factors. The current results represent an interesting case of antagonistic pleiotropy (i.e., certain genes are functional in some conditions, but could be deleterious in others) and adaptive gene loss with clinical importance. In Aim 1, we will apply QTL mapping methods and several alternative yeast genetic tools to identify other genes responsible for AmB resistance in YJM789. Interactions among contributing genes will also be quantified. In Aim 2, we will analyze the individual functions of the QTL genes to provide a mechanistic explanation for AmB resistance. A specific focus will be on determining why deletion of PDR5, a contributing genetic factor we have already identified, increases AmB resistance in yeasts. We hypothesize that PDR5 and its paralogous genes are involved in ergosterol homeostasis, which will be tested in S. cerevisiae and in a pathogenic yeast species, Candida albicans. The function and evolution of other genes identified in Aim 1 will also be investigated. In Aim 3, we will identify additional QTLs that are linked to drug resistance. More specifically, we will phenotype our panel of haploid progeny under a library of different small molecule drugs. Genetic loci which are responsible for the unique growth phenotypes of YJM789 in these environments will be identified using methodologies developed in Aim 1. These data will be used to test the hypotheses that the same types of drugs have similar functional mechanisms on pathogenic yeast growth and that mutations on certain regions of the genome can enable pathogenic yeasts to cope with exogenous small molecules.